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Digital To Analog Conversions

IP.com Disclosure Number: IPCOM000075418D
Original Publication Date: 1971-Sep-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 4 page(s) / 54K

Publishing Venue

IBM

Related People

Van Winkle, HE: AUTHOR [+2]

Abstract

Digital-to-analog converters (DAC's) are used in servo systems wherein error information signals supplied by device 10, being controlled over cable 11, are in digital form. Digital unit 12 receives the information and supplies true and complement (+ and -, respectively) digital-indicating signals. These signals include sign, S, and an ordered set of numerical digits 20 through 2/N/. Gating means, not shown, later described, connect digital unit 12 to the respective constant-current digit drivers 13. True and complement digit drivers 13 are provided. Input terminals 14 of true digit drivers 13 are, respectively, connected to the positive output terminals of the ordered set 20 through 2/N/. The negative outputs of digital unit 12 are, respectively, connected to terminals 15 in the same manner.

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Digital To Analog Conversions

Digital-to-analog converters (DAC's) are used in servo systems wherein error information signals supplied by device 10, being controlled over cable 11, are in digital form. Digital unit 12 receives the information and supplies true and complement (+ and -, respectively) digital-indicating signals. These signals include sign, S, and an ordered set of numerical digits 20 through 2/N/. Gating means, not shown, later described, connect digital unit 12 to the respective constant-current digit drivers 13. True and complement digit drivers 13 are provided. Input terminals 14 of true digit drivers 13 are, respectively, connected to the positive output terminals of the ordered set 20 through 2/N/. The negative outputs of digital unit 12 are, respectively, connected to terminals 15 in the same manner. Each digit driver 13 supplies identical constant-currents. Converting network or resistive ladder 20 weights the digit positions by binary resistive current dividing; the least significant digit positions are furthest from the output. The summed currents from the respective true and complement digit drivers are supplied to opposite inputs 24, 32 of operational differential amplifier (OP AMP) 21 via impedance matching network 22. Compensation or feedback shaping network 23 connects the output of OP AMP 21 to minus input at 24. Compensation network 23 is selected to shape the output of OP AMP 21 in accordance with characteristics and functions of device 10. Network 23 is polarity sensitive for compensating asymmetrical gain of OP AMP 21.

All of the digit drivers, both true and complement, share common power supplies such that power-supply drifts are compensated in OP AMP 21. The current-source voltage supply, +V1, is connected via identical current- determining resistors 25. All constant-current transistors 26 are biased to constant-current gain by reference potential +V2 supplied over terminals 27. Variation of +V2, likewise, is compensated at OP AMP 21. The collectors of transistors 26 are connected to resistive ladder 20 via isolating or switching diodes 28. The constant-current digit drivers are selectively decoupled from network 20 by control transistors 29. A suitable input signal on terminals 14 or 15 switches transistors 29 between current conduction and nonconduction for selectively shunting the digit constant-current to ground reference potential. When control transistors 29 are current nonconductive, the constant current is supplied to resistive ladder 20. Other switching means may be provided.

Resistive ladder 20 consists of two independent, but commonly grounded, current-summing resistive ladder portions. Adjacent the least significant digit positions 2 is a pair of terminating resistors 30 each having a resistance value R1. This same resistance value R1 is used to connect output terminals of adjacent drivers, as can be seen in the figure. Intermediate connections, such as at 31, are electrically connec...